1. Field of the Invention
This invention relates in general to methods and systems that transmit and receive audio and more particularly, that rely on multiband excitation vocoders to do so.
2. Description of the Related Art
In recent years, portable electronic devices, such as cellular telephones and personal digital assistants, have become commonplace. Many of these devices include a vocoder, such as a multiband excitation (MBE) vocoder. An MBE vocoder is a device that converts analog speech waveforms from various individuals into digital signals. These digital signals are then typically transmitted to another portable electronic device, where they are decoded and broadcast through a speaker to a user of the receiving portable electronic device.
Many MBE vocoders, however, have a limited encoding range. For example, most MBE vocoders are only able to encode speech waveforms that have pitch values between 80 Hz and 500 Hz. The range is limited because the vocoder is provided with a relatively small number of bits to cover the whole spectrum of pitch values generated by the different types of user voices (only a small number of bits are provided to preserve bandwidth).
Generally, the limited range is suitable for encoding the many different types of user voices. The pitch values of certain voice types, however, may exceed the encoding range of the vocoder. For example, the pitch values of the voice of a woman or a small child may surpass this range, particularly if the woman or small child is in an excited state. That is, the pitch inflections of certain individuals may exceed an allowable pitch range. In this instance, the vocoder cannot properly encode the speech waveforms, which will result in a degradation of voice quality.